1. Field of the Invention
The present invention relates to non-volatile semiconductor memories and, more particularly, to erasable programmable read-only memories with a large storage capacity.
2. Description of the Related Art
With the increasing needs for high performance and reliability of digital computer systems, the development of a semiconductor memory has strongly been desired which is large in storage capacity enough to replace an existing non-volatile data storage device, such as a magnetic floppy disk unit, for computer systems. Although being high in reliability, faster with respect to in data reading and writing speed and so on, presently-available electrically erasable programmable semiconductor read-only memories are not large enough in data storage capacity to replace the magnetic floppy disk unit.
In a conventional electrically erasable programmable read-only memory (EEPROM), each memory cell is typically comprised of two transistors, and the data write/erase operation is performed randomly one byte at a time. Therefore, the type of high-density integration that will provide a storage capacity large enough to replace peripheral data storage devices might be difficult to achieve.
As a non-volatile semiconductor memory of large capacity, an erasable programmable read-only memory with "NAND type cell" structure is proposed in "VLSI Symposium", R. Stewart et al., RCA, 1984, pp. 89-90. According to this reference the EEPROM shown only requires that each memory cell use a single transistor, and a single contact be provided between an array of memory cells arranged on a substrate to constitute an "NAND cell" structure and a corresponding bit line. Compared to the conventional EEPROMs, therefore, the area occupied by memory cells in the substrate can be reduced, improving the integration density.
However, a problem with in the area of above EPROM is the low reliability of operation. That is, when data is written into a selected cell, a non-selected cell (cells) adjacent to the selected cell may be brought into electrically unstable conditions. As a result, data may erroneously be written into the non-selected cell (cells). Such an erroneous write of data into the non-selected cell or cells would considerably degrade the reliability of the EEPROM, thus offering a serious problem.